1. Field of the Invention
The invention relates to a forme cylinder of a rotary printing press, in particular of an offset printing press.
2. Description of the Related Art
The forme cylinder can be used, in particular in offset printing, as a carrier or holding element for a printing forme which is plate-shaped and can be fixed in rounded form in axially extending clamping channels, or for a printing forme which is sleeve-shaped and can be pushed on in this form, but it is not restricted to this. Here, the printing unit can look like a conventional offset printing unit with a forme cylinder, i.e. printing-forme cylinder, as well as a blanket cylinder and an impression cylinder.
Thus, for example, DE 44 36 973 A1 describes a lithographic offset recto and verso press which has, in the printing unit, an upper and lower blanket cylinder and an upper and lower plate cylinder, which are each in rolling contact with a blanket cylinder at the printing nips.
In rotary offset presses of this type, as is known the printing image is transferred from the plate cylinder onto the blanket cylinder and from the latter onto the paper running over the impression cylinder. The ink can only be transferred, both from the plate onto the rubber blanket and also from the rubber blanket onto the paper, if a certain minimum pressure is present, what is referred to as the line pressure between the blanket cylinder and forme cylinder.
The printing-forme cylinder (i.e. the printing forme) thus fulfills the desired function, i.e. a constant print quality, permanently only if the setting (pressure) is correct. Too low a setting leads to the non-uniform transport of ink or dampening solution, because of the tolerances of circular running and cylindricity. Too high a setting has a detrimental effect on the service life of the cylinder surface, because of the internal friction and pressure overload.
In conjunction with this, a problem arises for quality assurance from the demand for ever higher productivity, and as a result of the efforts to produce printing-forme cylinders which are as light and cost-effective as possible. Especially what is referred to as channel-less printing, in particular therefore the sleeve technique which is distinguished by a printing forme applied without a seam onto a sleeve or a printing plate which is welded by laser to give a round shape, allows the rigidity to be reduced because of the lessened oscillation excitation as a result of the missing cylinder channels. As a result, the length-to-thickness ratio of the impression cylinders, or their relative rigidity with regard to deflection, becomes ever more unfavorable. The consequence of this is that, during printing operation, the shape and position of the impression cylinders with respect to one another change in an undesired manner, i.e. the impression cylinders are deflected.
Even when conventional plate cylinders are used, however, the impression-cylinder contour is pressed into the rubber blanket and the associated surface loading causes the cylinders to deflect, which leads to higher edge loading and to considerably lower loading in the center of the cylinders.
It is not possible to compensate for this lower loading in the cylinder center by increased cylinder pressure, because the edge zones would primarily profit from this and the central zones would only profit to a lesser extent.
The positional change as a consequence of a deflection changes the printing pressure, i.e. the setting pressure between the impression cylinders interacting in the printing unit, the pressure becoming non-uniform as seen across the cylinder width. This printing pressure is usually determined in numerical values by measuring what is referred to as the imprint width, i.e. the width of the zone which defines the contact area of the cylinders when the cylinders are thrown onto one another, i.e. moved to the pressure position. This measurement is particularly simple in offset printing, since here one cylinder of a pair of cylinders always has a compressible (soft) surface.
The printing or transfer characteristic curve, i.e. the tonal value gain, then depends directly on this imprint width, an increased imprint width meaning an increased tonal value gain and vice versa. The effect described therefore leads to a printing characteristic curve which changes in an undesired manner as viewed across the cylinder width.
In order to stabilize these printing characteristic curve values that vary across the cylinder width, previously either an appropriate support has been placed under the rubber blanket or, for example in the above-described DE 44 36 973 A1, the profile of the rubber blanket, i.e. its thickness, has been varied by the circumferential surface, in the axial direction of the cylinder, assuming a convex or concave shape on the blanket cylinder. Although the deflection between a blanket cylinder and a plate or forme cylinder can be compensated for by a convex profile of the blanket-cylinder surface, the contact is impaired, on the other hand, between the two blanket cylinders in a printing unit for recto and verso printing. This has a negative influence on both the web transport and the ink transfer to the paper web. Although the contact between the two blanket cylinders in the press nip is improved by a concave blanket-cylinder surface, the ink transfer from the forme cylinder to the blanket cylinder is then impaired.